Automotive and light commercial air conditioning systems generally use either a piston type or a scroll compressor. In operation, the scroll compressor differs from the piston compressor in that the volumetric efficiency of the scroll compressor goes up as rotational speed is increased while the volumetric efficiency of the piston compressor goes down as rotational speed is increased. Accordingly, the scroll compressor produces very high head pressures when a vehicle is accelerated in hot ambient temperature. Thus, the high side pressure switch may trip, causing system cooling performance to suffer.
While the piston compressor may produce 30,000 BTU/hr at high rotational speeds, the scroll compressor may produce 50,000 BTU/hr. Air conditioning systems that incorporate a suction accumulator aggravate the challenges with the scroll compressors due to violent boiling of the accumulator refrigerant when suction pressure is suddenly dropped by the scroll pumping capacity on acceleration of the vehicle. This results a greater mass flow rate to the compressor as compared to a thermostatic expansion valve system with no accumulator. While a suction throttling valve may solve this problem, most are cost prohibited and complex to install.
Sonic nozzles have been used for years in fluid flow measurement. At flow rates below sonic, the nozzle can recover up to 94% of pressure loss. In comparison, a square edge orifice recovers only 12% of pressure loss. Pressure recovery is essential as each 1.0 psi drop in suction plumbing reduces capacity by approximately 1%. A ¼ inch diameter square edged orifice may produce a 7 psi drop at a given flow rate while the sonic nozzle may be 1 psi at similar flow and throat size.